nutrient mineralization and storage in ecosystems. Here we ask to what degree
does variation in animal size distribution regulate nutrient mineralization?
Researchers have described a wide variety of biomass-size distributions (also
called size spectra) for aquatic animal assemblages, including flat or smooth,
uni-, bi- and poly-modal, and step or asymptotic functions. Size distributions
can vary considerably in space and time within and among aquatic habitats
(Hanson, Prepas & Mackay,1989 ;Steadet al., 2005 ), complicating generalizations
(see Warwick, this volume). The diversity of methods in body-size estimations (for
example, Morin & Nadon,1991 ; Ramsayet al., 1997 ; Baca & Threlkeld,2000 )and
analytical techniques, such as different sieve or size classes, further complicate
size-spectra summaries (Cyr & Pace,1993 ; Robson, Barmuta & Fairweather,2005 ).
However, when only the invertebrate portions of published aquatic assemblage
spectra are included (that is, smaller and larger portions excluded), clearer pat-
terns of shape categories emerge. Most size spectra have biomass peaks that are
skewed left, meaning larger animals generally account for most of the total
biomass, even though they may be outnumbered by smaller ones.
Size distributions in lakes vary as a function of habitat. Studies that include
multiple habitat types within the same lake suggest that pelagic and littoral
assemblages tend to have bimodal distributions of invertebrates (Hansonet al.,
1989 ; Cyr & Pace,1993; Rasmussen, 1993 ) and polymodal distributions when
fishes are included (Gaedke, 1992 ), whereas profundal (and sublittoral) distri-
butions tend to be unimodal (Hansonet al., 1989). The magnitude and locations
of biomass peaks and troughs also vary among habitats within lakes; littoral
habitats have peaks at larger body sizes. For example, the two biomass maxima
for littoral habitats tended to occur between 1–4mg and 64–256 mg wet mass
(Rasmussen,1993), whereas the two peak densities of pelagic zooplankton
occurred between 0.044–0.125mg and 2.0–11.3mg dry mass for small and large
animals, respectively (Cyr & Pace, 1993 ).
Streams generally have unimodal biomass size distributions (Cattaneo,1993;
Bourassa & Morin, 1995 ; Mercieret al., 1999; Schmid, Tokeshi & Schmid-Araya,
2002 ). Body-size maxima, as equivalent to a spherical diameter, were between
2–4 mm in streams (Cattaneo,1993), and the average individual biomass
increased slightly with increasing trophic status from 24–40mg dry mass in
oligotrophic to urban eutrophic streams, respectively (Bourassa & Morin,
1995 ). Overall, although unimodality is robust across many streams, total bio-
mass can vary by an order of magnitude (for example, Bourassa & Morin,1995)
suggesting possible dramatic differences in animal driven nutrient fluxes
within a stream system.Estimating nutrient flux from biomass size distributions
Animal assemblages with different size distributions should have different
nutrient supply rates to ecosystems, all else being equal. To illustrate this294 R.O. HALLET AL.